Polishing processes are used in a variety of technologies and for a number of purposes. For many applications, the polish is for aesthetic or mechanical purposes and the microscopic accuracy of the polish is not critical. However, in some applications, such as for processing of electronic materials and/or components, it is important that the polishing process be accurate. For example, an uneven or overly deep polish may ruin some or all of a product, such as a wafer bearing one or more finished or intermediate integrated circuits. On the other hand, an insufficiently deep polish, even if uniform, may render the product unsuitable as well. Accordingly, for many applications, it is necessary that polishing be quite accurate.
A number of prior art methods are available to address this problem. For example, it is known to perform ex situ monitoring of a polishing process. An example of this technique involves removing a piece being polished from the polishing process periodically and using tests to determine the extent and quality of the polish process at that point. Typically this technique is used beforehand to develop a polishing protocol rather than to check each piece during actual production. This assumes a great degree of consistency and control with respect to the polish process parameters.
Moreover, such techniques are expensive, slow, and can be inexact. The expense and slowness arise due to the need to perform multiple experiments and to repeatedly start and stop the polishing process. The inexact nature of this technique is due to the fact that often no appropriate measurement or monitoring occurs during the actual process of interest, i.e. during the production process. Thus, variations in any number of factors may affect the polish rate and/or quality without any ability to detect and correct such variations in real time.
A technique sometimes used to detect the endpoint of an ongoing polishing step involves the monitoring of the frictional force between the wafer and a polishing pad. When the frictional force changes suddenly, it is assumed that the previous layer has been removed, and that a new layer, with a different frictional coefficient, has been exposed. However, this procedure assumes that the materials involved have significantly different frictional coefficients. Moreover, even if the frictional coefficients are substantially different from one another, it is still often challenging to detect the small changes in force. On the whole, the practicality and accuracy of this technique are lacking.
A more common technique for in situ surface analysis involves laser interferometry. Using this technique, a hole or window is typically placed in the polishing pad and laser radiation is directed through the window onto the polished surface. The reflection of the laser radiation from the polished surface is collected and analyzed to determine the thickness of the top layer. The reflected light will typically comprise a component that has penetrated the surface before reflecting as well as a component that has reflected from the surface without penetrating. The path difference between these components yields an oscillating (interference) pattern in the collected reflection, which can then be processed to track the layer thickness.
This technique, while somewhat effective, has a number of shortcomings. For example, since the technique requires that a hole be made in the polishing pad, the possibility of leakage, and consequent disruption of the polishing process, is increased. Moreover, the technique can only be used to analyze an entire layer with all of its components, and cannot be used to analyze just one of multiple surface constituents. Furthermore, since the pad is often rotating or oscillating, it is only possible to obtain an intermittent interferometric reading. This is especially troublesome near the end of a polishing process, when a lag on the order of a second or so may be important. In addition, the presence of the hole in the polishing pad can alter the behavior of the polishing action. Finally, interferometric measurements can become unreliable towards the end of a polishing step as the layer under analysis becomes infinitely thin.
As a result of the deficiencies in existing techniques, the rate of defective products is higher, resulting in lower yield and higher cost, than would be attainable if an effective in situ monitoring process were available. Moreover, the development of new polishing procedures would be faster and more efficient if a practical in situ process monitoring system were available.